Add feature flags for Hub/Spoke resources

[JinLee794]

Customers have use-cases that may involve bringing their own resources (for example, an existing hub configuration)

Would like to refactor the bicep/terraform templates to accommodate for optional infrastructure resources being created and give the user set number of options on types of existing resources they can bring in.

[JinLee794]

• bicep template: jumpbox vm should be named to be more descriptive instead of (vm-<app_name>-<env>) as well as the self-hosted agent (instead of just github-<env>)

[thomas-tran]

Could we add the flag whether or not hub is required in the deployment? Customer may only need the spoke vnet and did not want to connect to any hub. Or at least, create a hub placeholder with/without firewall or bastion